This invention relates to measuring cardiac output and, more particularly, to a method and means of continuously measuring cardiac output.
Many medical procedures include the monitoring of cardiac output (the rate of blood flow through the heart). A widely used method is to place a catheter into the right side of the heart through a major vein to position a temperature sensor, such as a thermistor, in the pulmonary artery. This catheter may also be placed to perform other functions such as pressure monitoring. To determine cardiac output, a fluid cooler than the blood, usually a glucose or saline solution, is injected down one of the lumens in the catheter and into the bloodstream in the vicinity of the right atrium. This results in a thermodilution temperature curve being sensed in the pulmonary artery by the thermistor. By controlling the volume and temperature of the injected fluid, recording the blood temperature prior to the thermodilution curve and integrating the thermodilution curve over time, an attached computer is able to calculate and display the cardiac output. Because these curves are of short duration, and the blood flow is subject to longer term variations, conventional practice is to take several measurements and average them. Reference may be made to U.s. Pat. No. 4,632,125, the disclosure of which is incorporated herein.
Unfortunately, the foregoing procedures result in a significant volume of fluid being injected into the patient's bloodstream. To prevent stressing the patient's system in a condition commonly referred to as "volume loaded," the frequency of cardiac output determinations must be limited. Many physicians and researchers desire to have much more frequent determinations to provide better patient care or more complete research data.
In addition, particular care must be taken that the injected fluid is sterile to avoid microorganism related problems being introduced into the patient. Since the injections are commonly made by syringe, the connection to the injectate lumen of the catheter must be broken and reconnected to the next syringe for the next injection. Often these syringes are prefilled and placed in an ice bath prior to use. Despite extreme attention to good sterile procedure, these operations still present a finite risk of contamination. To respond to this risk, some catheter manufacturers have introduced systems which allow the use of a single syringe, and cool the fluid in a heat exchanger prior to its being routed to the syringe. These systems have not gained wide acceptance for a variety of reasons. Chief among them are, the clumsiness of such systems, accuracy questions due to the lack of control of the injectable temperature and its monitoring, and the difficulty of maintaining the sterility of the inner surface of the syringe.
Other dilution techniques using indicators such as cardio-green dye suffer from the same kind of problems. In addition, they suffer from a recirculation of the indicator that complicates the procedure and also limits the acceptable rate at which determinations of cardiac output may be made. This accounts for their lack of popularity.
To eliminate these problems, many systems have been proposed, but only a few have met with even limited success. Completely non-invasive ultrasonic techniques are available, but require expensive equipment and skilled operators. Also the required placements of external devices on the patient preclude their use during some surgical procedures or in the case where the patient may move during a determination. The electromagnetic flow meter technique has been incorporated into a catheter to eliminate the previously required surgical procedures to implant and remove the necessary sensor. However, the present size of the sensor on the catheter precludes the use of the popular "sterile sheath" systems of catheter insertion. Thus a cut-down must be performed, at a greater risk to the patient. Also, current design does not allow useful and desired pressure monitoring to be done. In addition, they are intended for reuse, which requires hospital cleaning and sterilization procedures and facilities that are often not feasible. Other catheter mounted velocity sensor systems have suffered from many design and theoretical problems that have not been resolved. Although they potentially can greatly reduce the sterility risks and dramatically increase the frequency that cardiac output determinations may be made, no system has yet been introduced.